It is known that the coherence of a laser beam is currently evaluated by measuring the proportion of energy of the beam which is inside a cone whose apex angle is called the angle of divergence.
But this measurement takes into account only the spatial divergence of the beam and it appears insufficient to study in particular the propagation of a laser beam.
A mutual coherence function of a laser beam has therefore been defined, said function characterizing in particular the time coherence of the beam, i.e. the perturbations and the fluctuations of the phase of the laser wave. This definition is set forth in chapter 10, paragraph 3, pages 499 to 505 of the book "Principles of Optics" By Max Born and Emil Wolf (published by Pergamon in 1964--Second revised edition).
The present invention aims to measure the time coherence of a laser beam.